1. Field of the Invention
The present invention relates to an automatically driven motor vehicle, and more particularly to an automatically driven motor vehicle capable of automatically running behind a front motor vehicle while controlling the distance between itself and the front motor vehicle.
2. Description of the Prior Art
Research efforts are being made to develop a technology for automatically driving motor vehicles such as automobiles on roads while detecting obstacles with a radar, a CCD camera, or their combination to recognize front obstacles and road conditions. However, there has not been available any technology, to be incorporated into automatically driven motor vehicles, for appropriately recognizing front obstacles and road conditions on any roads.
Automatic vehicle travel control under given conditions has already been practiced in limited applications such as automatic transport carriage control in factories, for example. Such an automatic transport carriage is controlled to travel at a low speed along a predetermined path based on the detection of magnetic markers that are arranged at given intervals along the path.
However, it is difficult to control the automatic transport carriage to run accurately on the predetermined path, and the actual position where the automatic transport carriage runs tends to be displaced in error from the predetermined path while the automatic transport carriage is running.
It has been customary to control the automatic transport carriage to run along the predetermined path by detecting a displacement error between the present position of the automatic transport carriage and the predetermined path and eliminating the displacement error under feedback control.
The conventional automatic transport carriage control process has been unstable because of hunting in eliminating displacement errors particularly when the automatic transport carriage runs at high speeds. It has been difficult to run the automatic transport carriage on general roads while controlling its speed.
There has been proposed an automatically driven motor vehicle which automatically follows a front motor vehicle by detecting the front motor vehicle and controlling the distance (intervehicular distance) between itself and the front motor vehicle with a function that is proportional to the speed of the front motor vehicle.
For example, as shown in FIG. 4(a) of the accompanying drawings, when a speed v of a front motor vehicle is detected at the time the front motor vehicle starts to move, an intervehicular distance L which is proportional to the speed V of the front motor vehicle is calculated according to the equation given belay and outputted as a command signal for the intervehicular distance L between the automatically driven motor vehicle and the front motor vehicle: EQU L=s.multidot.V
where s is a coefficient expressed in time.
An acceleration or deceleration for the automatically driven motor vehicle (the following motor vehicle) to keep the intervehicular distance L is determined, and the speed of the automatically driven motor vehicle is controlled according to the determined acceleration or deceleration.
After the acceleration of the automatically driven motor vehicle is finished and the front motor vehicle runs at a constant speed, the automatically driven motor vehicle runs at the same speed as the speed of the front motor vehicle while keeping an intervehicular distance corresponding to its speed.
If the front motor vehicle is abruptly decelerated or a motor vehicle which happens to be stopping due to a traffic jam or a breakdown is recognized as the front motor vehicle, then the command signal for the intervehicular distance is abruptly reduced. Therefore, the automatically driven motor vehicle (the following motor vehicle) controls its speed to abruptly reduce the intervehicular distance between itself and the front motor vehicle.
As a consequence, the following motor vehicle is decelerated at a ratio smaller than the front motor vehicle, and runs abruptly close to the front motor vehicle.
As shown in FIG. 4(b) of the accompanying drawings, such a drawback can be eliminated by producing command signals for intervehicular distances Ls, Le (&gt;0), as indicated below, when the speed V is zero (V=0) at the time the following motor vehicle starts to move, i.e., starts to be accelerated, and comes to a stop, i.e., is decelerated, respectively. EQU L=Ls+s.multidot.V (when the motor vehicle starts to move), EQU L=Le+s.multidot.V (when the motor vehicle comes to a stop)
Because the command signal for the intervehicular distance L is always larger than Ls or Le, the automatically driven motor vehicle is prevented from getting too close to the front motor vehicle even when the front motor vehicle is abruptly decelerated.
However, the above proposal suffers its own drawback. Since the intervehicular distance L represented by the above equations is present between the front motor vehicle and the following motor vehicle when the speed is of a certain value, the intervehicular distance L includes an extra distance equal to Ls or Le and hence is unnecessarily greater than actually required.
As represented by the above equations, the intervehicular distances Ls, Le at the time the speed V is nil are different from each other when the following motor vehicle starts to move and comes to a stop, respectively, and the command signals are determined by respective different functions. Consequently, while the motor vehicle is running at a certain speed Vc (see FIG. 4(b)), for example, the functions representing the respective command signals need to be switched over. As a result, the actual command signal tends to change discontinuously from one level to another, causing the speed V to change abruptly.